1. Technical Field
The invention generally relates to laser detection and ranging.
2. Description of the Related Art
Currently, there are a number of systems for performing laser detection and ranging (LADAR). Typically, the photocurrent from the detector of a LADAR system is amplified with a high-gain amplifier because the power of the received light signal and the resulting output current of the LADAR receiver is relatively small (e.g., sub-nanoamp currents). The use of high-gain amplifiers, however, may be problematic because of resulting noise. For instance, in a LADAR system, the noise resulting from the high-gain amplifier may limit the sensitivity of the LADAR receiver.
One existing solution for overcoming amplifier noise is to use a high-gain detector, such as an avalanche photodiode (APD). APDs, however, may also introduce excess noise into the LADAR system. When modulating the APD gain to produce photonic-mixing, as required for the chirped amplitude modulation (AM) LADAR technique, the modulated excess noise may produce large amounts of self-clutter. Some chirped AM LADAR systems have reduced this problem by using metal-semiconductor-metal (MSM) photonic-mixing detectors. Such systems have been effective for 3-D imagery and have improved the self-clutter problem, but with only near unity gain, so that the receiver is amplifier-noise limited.
Some pulsed time-of-flight LADAR systems employ photon counting detectors such as high gain photomultiplier tubes (PMT), high gain microchannel plate (MCP) based image intensifier tubes, intensified photodiodes (IPD), or Geiger-mode APDs to overcome the excess APD noise and amplifier noise. For example, when operated with a reverse bias voltage above the breakdown voltage, a Geiger-mode APD outputs a large pulse upon single carrier excitation (e.g., single photon detection or dark noise carrier excitation). Typical gains in these types of LADAR receivers, for example, are more satisfactory. However, there is the additional problem that the output pulse voltage/current level is not related to the received optical power level in Geiger mode operation. Since range information is carried on the modulation of the optical power level in chirped AM LADARs, photon counting detectors have not previously been used as detectors for chirped AM LADARs.